We are in the midst of a worldwide epidemic of obesity and related metabolic diseases. It is now known that the incidence type 2 diabetes, NASH, cardiovascular disorders and even cancer are increased with obesity. This represents a huge burden upon the health and health care system of the United States and much of the rest of the world. While education concerning diet and exercise are critical components to a solution for these problems, there is a huge need to improve both our basic understanding of and therapeutic options for all of these conditions. Two key parts of organismal energy balance are food intake and energy expenditure. The latter includes physical activity and brown fat-mediated thermogenesis. Our work under this grant (over several years) discovered the PGC1 transcriptional coactivators and their role in various pathways of mitochondrial biogenesis and oxidative metabolism. In the last grant cycle we identified 3 new proteins encoded by the PGC1? gene; PGC1?4, in particular, has interesting and powerful activities in muscle and brown fat. PGC1?4 does not stimulate mitochondrial biogenesis in muscle, but rather increases muscle hypertrophy, strength and resistance to cancer cachexia. Preliminary data here shows that it affects UCP1 and other themogenic genes in fat but again, does not stimulate mitochondrial biogenesis. PGC1?4 also increases expression of a novel myokine, meteorin-like (METRNL), that is a powerful stimulator of adipose tissue browning through modulation of the immune cells within fat tissue. The new cycle of this grant will explore the full range of biologicl functions of METRNL in cellular and systemic metabolism, using knock-out mice already breeding in our lab. Preliminary data has identified a clonal fat cell line that has a molecular response to METRNL and we will use these cells (3T3-F442A) to clone and characterize the METRNL receptor. Isolation of this receptor will be through protein purifications, using tagged MERTNL and advanced protein Mass Spectrometry methods, specifically isobaric tagging. It is expected that the receptor identification will also lead us to new tissues and pathways where METRNL may function in important ways. Lastly, we will return to a key mechanistic problem: how can PGC1?4, a shorter form of the canonical PGC1? protein, regulate such a different set of genes in both muscle and brown fat. Because the PGC1s are coregulatory proteins, this must involve, at least in part, differential interactions with partner transcription factors and other proteins. To investigate this, we will use purifications of the PGC1? complexes, followed by comparative and quantitative Mass Spectrometry. The function of these new factors in the PGC1 complexes will be studied by genetic and (potentially) pharmacological methods.